查看更多>>摘要：We investigate the anisotropic band structure and its evolution under tensile strains along different crystal-lographic directions in bulk black phosphorus(BP)using angle-resolved photoemission spectroscopy and density functional theory.The results show that there are band crossings in the Z-L(armchair)direction,but not in the Z-A(zigzag)direction.The corresponding dispersion-k distributions near the valence band maximum(VBM)exhibit quasi-linear or quadratic relationships,respectively.Along the armchair direction,the tensile strain expands the interlayer spacing and shifts the VBM to deeper levels with a slope of-16.2meV/％strain.Conversely,the tensile strain along the zigzag direction compresses the interlayer spacing and causes the VBM to shift towards shallower levels with a slope of 13.1 meV/％strain.This work demonstrates an effective method for band engineering of bulk BP by uniaxial tensile strain,elucidates the mechanism behind it,and paves the way for strain-regulated optoelectronic devices based on bulk BP.
查看更多>>摘要：Hybrid skin-topological effect(HSTE)in non-Hermitian systems exhibits both the skin effect and topological protection,offering a novel mechanism for localization of topological edge states(TESs)in electrons,circuits,and photons.However,it remains unclear whether the HSTE can be realized in quasicrystals,and the unique structure of quasicrystals with multi-site cells may provide novel localization phenomena for TESs induced by the HSTE.We propose an eight-site cell in two-dimensional quasicrystals and realize the HSTE with eight-site nonreciprocal intracell hoppings.Furthermore,we can arbitrarily adjust the eigenfield distributions of the TESs and discover domain walls associated with effective dissipation and their correlation with localization.We present a new scheme to precisely adjust the energy distribution in non-Hermitian quasicrystals with arbitrary polygonal outer boundaries.
查看更多>>摘要：Determination of the magnetic structure and confirmation of the presence or absence of inversion(P)and time reversal(T)symmetry is imperative for correctly understanding the topological magnetic materials.Here high-quality single crystals of the layered manganese pnictide CaMnSb2 are synthesized using the self-flux method.De Haas-van Alphen oscillations indicate a nontrivial Berry phase of～π and a notably small cyclotron effective mass,supporting the Dirac semimetal nature of CaMnSb2.Neutron diffraction measurements identify a C-type antiferromagnetic structure below TN=303(1)K with the Mn moments aligned along the a axis,which is well supported by the density functional theory(DFT)calculations.The corresponding magnetic space group is Pn'm'a',preserving a P × T symmetry.Adopting the experimentally determined magnetic structure,band crossings near the Y point in momentum space and linear dispersions of the Sb 5py,z bands are revealed by the DFT calculations.Furthermore,our study predicts the possible existence of an intrinsic second-order nonlinear Hall effect in CaMnSb2,offering a promising platform to study the impact of topological properties on nonlinear electrical transports in antiferromagnets.
查看更多>>摘要：We propose a method to construct Hopf insulators based on the study of topological defects from the geometric perspective of Hopf invariant I.Firstly,we prove two types of topological defects naturally inhering in the inner differential structure of the Hopf mapping.One type is the four-dimensional point defects,which lead to a topological phase transition occurring at the Dirac points.The other type is the three-dimensional merons,whose topological charges give the evaluations of I.Then,we show two ways to establish the Hopf insulator models.One approach is to modify the locations of merons,thereby the contributions of charges to I will change.The other is related to the number of defects.It is found that I will decrease if the number reduces,while increase if additional defects are added.The method developed in this study is expected to provide a new perspective for understanding the topological invariants,which opens a new door in exploring and designing novel topological materials in three dimensions.
查看更多>>摘要：The nontrivial band topologies protected by certain symmetries have attracted significant interest in condensed matter physics.The discoveries of nontrivial topological phases in real materials provide a series of archetype materials to further explore the topological physics.Ternary borides M2XB2(M=W,Mo;X=Co,Ni)have been widely investigated as the wear-resistant and high-hardness materials.Based on first-principles calculations,we find the nontrivial topological properties in these materials.Taking W2NiB2 as an example,this material shows the nodal line semimetal state in the absence of spin-orbit coupling.Two types of nodal lines appear near the Fermi level simultaneously.One is protected by the combined space-inversion and time-reversal symmetry,and the other is by the mirror symmetry.Part of these two-type nodal lines form nodal chains.When spin-orbit coupling is included,these nodal lines are fully gapped and the system becomes a strong topological insulator with nontrivial Z2 index(1;000).Our calculations demonstrate that a nontrivial spin-momentum locked surface Dirac cone appears on the((1)10)surface.We also find that other isostructural ternary borides Mo2NiB2,Mo2CoB2,and W2CoB2 possess similar topological band structures.Therefore,our work not only enriches the understanding of band topology for ternary borides,but also lays the foundation for the further study of topological phases manipulation and potential spintronic applications in realistic materials.
查看更多>>摘要：SnO2 films exhibit significant potential as cost-effective and high electron mobility substitutes for In2O3 films.In this study,Li is incorporated into the interstitial site of the SnO2 lattice resulting in an exceptionally low resistivity of 2.028 × 10-3 Ω·cm along with a high carrier concentration of 1.398 × 1020 cm-3 and carrier mobility of 22.02 cm2/V·s.Intriguingly,Lii readily forms in amorphous structures but faces challenges in crystalline formations.Furthermore,it has been experimentally confirmed that Lii acts as a shallow donor in SnO2 with an ionization energy ΔED1 of-0.4 eV,indicating spontaneous occurrence of Lii ionization.
查看更多>>摘要：Moiré superlattices in twisted two-dimensional materials have emerged as ideal platforms for engineering quantum phenomena,which are highly sensitive to twist angles,including both the global value and the spatial inhomogeneity.However,only a few methods provide spatial-resolved information for characterizing local twist angle distribution.Here we directly visualize the variations of local twist angles and angle-dependent evolutions of the quantum states in twisted bilayer graphene by scanning microwave impedance microscopy(sMIM).Spatially resolved sMIM measurements reveal a pronounced alteration in the local twist angle,approximately 0.3° over several micrometers in some cases.The variation occurs not only when crossing domain boundaries but also occasionally within individual domains.Additionally,the full-filling density of the flat band experiences a change of over 2 × 1011 cm-2 when crossing domain boundaries,aligning consistently with the twist angle inhomogeneity.Moreover,the correlated Chern insulators undergo variations in accordance with the twist angle,gradually weakening and eventually disappearing as the deviation from the magic angle increases.Our findings signify the crucial role of twist angles in shaping the distribution and existence of quantum states,establishing a foundational cornerstone for advancing the study of twisted two-dimensional materials.
查看更多>>摘要：The H+NaF reaction is investigated at the quantum state-resolved level using the time-dependent wave-packet method based on a set of accurate diabatic potential energy surfaces.Oscillatory structures in the total reaction probability indicate the presence of the short-lived intermediate complex,attributed to a shallow poten-tial well and exothermicity.Ro-vibrational state-resolved integral cross sections reveal the inverted population distributions of the product.The HF product favors an angular distribution in the forward hemisphere of 30°-60° within the collision energy range from the threshold to 0.50 eV,which is related to the nonlinear approach of the H atom to the NaF molecule.Quantum generalized deflection functions show that the low-J partial waves con-tribute primarily to the backward scattering,while the high-J partial waves govern the forward scattering.The correlation between the partial wave J and the scattering angle & proves that the reaction follows a predominant direct reaction mechanism.
查看更多>>摘要：Polymer-liquid crystals(PLCs)are common materials for smart windows.However,PLC smart windows usually require high driving voltage to maintain transparency.We synthesized a novel PLC smart film by doping multi-wall carbon nanotubes(MWCNTs)into a reverse-mode polymer network liquid crystal(R-PNLC).It is found that doping MWCNTs could effectively reduce the threshold voltage(Vth)of R-PNLC from 19.0 V to 8.4 V.Due to co-orientation between MWCNT and LC molecules,the doped R-PNLC is able to maintain a high transmittance of visible light(～80％)without an applied electric field.We find that doping MWCNTs could change the frequency modulation property of R-PNLC.The doped R-PNLC exhibits a wider frequency modulation range up to 40000 Hz,while the frequency modulation of the undoped R-PNLC reached to a saturation at 23000 Hz.We also tested the electromagnetic interference(EMI)shielding efficiency of R-PNLC and find that the EMI shielding efficiency could be improved by doping only 0.01 wt％MWCNTs into the system.The total shielding effectiveness of 0.01 wt％MWCNT doped R-PNLC was up to 14.91 dB in the frequency band of 5.38-8.17 GHz.This study demonstrates that the films are potentially useful for low-energy-consumption smart windows with enhanced electromagnetic shielding capability.
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